EP2064151B1 - Improved method for producing prussic acid - Google Patents

Abstract

The invention relates to a method for the production of prussic acid, comprising the catalytic dehydration of gaseous formamide to give a product mixture, containing prussic acid, water, formamide, ammonia, CO and high-boilers, separation of the product mixture by condensation of a condensate from the product mixture which contains water, formamide, high-boilers and possibly ammonia and prussic acid, and the partial or complete recycling of the condensate to a heat exchanger downstream of the reactor.

Description

The present invention relates to a process for the preparation of hydrocyanic acid comprising the catalytic dehydration of gaseous formamide to give a product mixture containing hydrocyanic acid, water, formamide, ammonia, CO and high boilers, separation of the product mixture by condensation of a condensate from the resulting product mixture containing water Formamide, high boilers and optionally ammonia and hydrocyanic acid, as well as the partial or complete recycling of the condensate in a reactor downstream of the heat exchanger.

Hydrocyanic acid is an important basic chemical that is used as a starting material, for example, in numerous organic syntheses such as the production of methacrylic acid esters, lactic acid and metal cyanides, as a polyamide precursor, for the production of pharmaceutical and agrochemical products, in the mining industry and in the metallurgical industry.

An important process for the industrial production of hydrocyanic acid is the thermal dehydration of formamide in vacuo, which proceeds according to the following equation (I):

This reaction is accompanied by the decomposition of the formamide according to equation (II) to form ammonia and carbon monoxide:

The ammonia formed catalyses the polymerization of the desired hydrocyanic acid and thus leads to an impairment of the quality of the hydrocyanic acid and a reduction in the yield of the desired hydrocyanic acid.

In the production of hydrocyanic acid, soot is formed in the reactor, which is accompanied by the addition of small amounts of oxygen in the form of air, as in EP-A 0 209 039 is disclosed, can be suppressed. In EP-A 0 209 039 discloses a process for the thermolytic cleavage of formamide on highly sintered alumina or alumina-silica moldings or on high-temperature corrosion-resistant chromium-nickel stainless steel moldings. The formamide conversion is not complete in this process. In order to increase the total formamide conversion, separation and recycling of unreacted residual formamide thus makes sense.

DE-A 101 38 553 relates to a process for the production of hydrogen cyanide by catalytic dehydration of gaseous formamide in the presence of a catalyst containing iron in the form of metallic iron and / or iron oxide. In particular, the catalyst is selected from packings of steel and iron oxides on a support material of alumina.

In WO 2004/050587 discloses a process for producing hydrocyanic acid by catalytic dehydration of formamide, which reactor has an inner reactor surface made of a steel containing iron and chromium and nickel. Preferably contains in the method according to WO 2004/050587 used reactor no additional installations.

WO 2006/027176 relates to a process for the production of hydrogen cyanide by catalytic dehydration of gaseous formamide, in which from the product mixture of the dehydration a formamide-containing reflux is separated and recycled to the dehydration, wherein the reflux contains 5 to 50 wt .-% water. It is preferred according to WO 2006/027176 condensed from the product gas mixture of the dehydration, a mixture of water, formamide, high boilers and dissolved gases as condensate 1, separated from the condensate 1, a portion of the water and the dissolved gases, a condensate 3 is obtained from water, formamide and high boilers, and the Formamide-containing recycle stream is separated from the condensate 3 by simple distillation.

In the separation of unreacted formamide and water of reaction, the crude gas is cooled and condensed formamide with a portion of the water. For this purpose, downstream of the reactor heat exchangers, e.g. Tube bundle heat exchanger used. At the points where condensed phases are formed, deposits of solids, e.g. consisting of polymeric HCN. As a consequence of this, the process for producing HCN has to be interrupted at regular intervals and the occupied apparatuses have to be cleaned. This condition results in a shutdown that leads to massive economic damage.

GB 1295379 describes the Catalan dehydration of gaseous formamide with following condensation. In the condensation, a condensed aqueous Blansäurestrom is obtained which is partially sprayed by spray nozzles on the condenser head.

It is therefore an object of the present invention to provide a process for preparing hydrogen cyanide by catalytic dehydration of formamide, which process is easy to carry out and which is characterized by high availability and short storage times.

1. i) katalytische Dehydratisierung von gasförmigem Formamid in einem Reaktor mit nachgeschaltetem Wärmetauscher mit Kondensationsflächen, wobei ein Produktgemisch erhalten wird, enthaltend Blausäure, Wasser, Formamid, Ammoniak, CO und Hochsieder;
2. ii) Teilkondensation einer flüssigen Phase, enthaltend Formamid und wassen und gegebenenfalls Ammoniak und Blansäure in geringer menge von 0,5 gewicht % unter erhalt eines gasförmigen produkt stroms, der Blansäure, wasser, Ammoniak und co enthält, aus dem Produktgemisch,
3. iii) kontinuierliche Rückführung eines Teils der auskondensierten Flüssigkeit und Zurückpumpen dieser auskondensierten Flüssigkeit auf die Kondensationsflächen des Wärmetauschers, wobei die gesamten kodensations flächen des nachgeschalteten Wärmetanschens ständing mit flüssigkeit benezit werden.

This object is achieved by a process for producing hydrocyanic acid comprising

1. i) catalytic dehydration of gaseous formamide in a reactor with downstream heat exchanger with condensation surfaces, whereby a product mixture is obtained, containing hydrocyanic acid, water, formamide, ammonia, CO and high boilers;
2. ii) partial condensation of a liquid phase containing formamide and wass and optionally ammonia and blended acid in a small amount of 0.5% by weight to obtain a gaseous product stream containing blanic acid, water, ammonia and co, from the product mixture,
3. iii) continuous recycling of a portion of the condensed liquid and pumping back this condensed liquid on the condensation surfaces of the heat exchanger, the entire kodensations surfaces of the downstream Wärmetanschens are ständing with liquid benezit.

This achieves two effects: on the one hand, the incoming hot process gas is cooled (quenched), for example, from about 450 ° C to about 250 ° C, and on the other hand by the partial return of the condensed liquid (condensate) on the condensation surfaces of the heat exchanger may be forming HCN polymer rinsed regularly from the heat exchanger surfaces. Caking or even clogging of the reactor by deposits on the heat exchanger surfaces can thus be delayed or almost completely prevented with the aid of the method according to the invention.

It is preferred that the recycled condensate is distributed as evenly as possible on the condensation surfaces of the heat exchanger. This can be achieved by passing the condensate at one or more points on the condensation surface. In this case, the condensate is carried on the condensation surfaces of the heat exchanger in general by the fact that the condensate is pumped by means of pumps on the condensation surfaces of the heat exchanger. In one embodiment of the method according to the invention, the condensate is pumped to the entry point of the heat exchanger. The preferred addition point is chosen so that the parts of the heat exchanger are constantly lapped, takes place at the phase transition gas / liquid. The distribution of the condensate on the condensation surfaces of the heat exchanger takes place in a preferred embodiment of the method according to the invention by means of one or more, generally arranged at different locations, nozzles. As a result, the recirculated condensate is finely and evenly distributed.

The entire condensation surfaces of the downstream heat exchanger are constantly wetted with liquid. The ratio of recycled condensate and crude gas (in parts by weight) is generally 1: 2 to 1:20, preferably 1: 3 to 1:15 and more preferably 1: 3 to 1:10.

Step i)

In step i) of the process according to the invention, the catalytic dehydration of gaseous formamide takes place.

Suitable catalysts are all known catalysts suitable for dehydration of gaseous formamide. Examples of suitable catalysts are highly sintered moldings composed of 50 to 100 wt .-%, preferably 85 to 95 wt .-% alumina and 0 to 50 wt .-%, preferably 5 to 15 wt .-% silica and chromium-nickel stainless steel Shaped body, as in EP-A 0 209 039 describes packages containing steel and / or iron oxide on a support material, as in DE-A 101 38 553 described. Furthermore, the inner wall surface of the reactor may be formed of steel, for example of V2A steel, and serve as a catalyst. It is not necessary that the reactor contains other internals. A suitable reactor in which the reactor wall serves as a catalyst is, for example, an empty tubular reactor of V2A steel, with the hot tube wall surface serving as a catalyst. Suitable reactors are in WO 2004/050587 described.

In step (i) of the process according to the invention, the gaseous formamide can be reacted in the presence of atmospheric oxygen. Usually 0 to 10 mol% atmospheric oxygen, based on the amount of formamide used, preferably 0.1 to 10 mol%, particularly preferably 0.5 to 3 mol% of atmospheric oxygen.

The temperature in step i) of the process according to the invention is generally from 350 to 650.degree. C., preferably from 400 to 600.degree. C., more preferably from 450 to 550.degree. C., very preferably from 500 to 550.degree.

Step i) of the process according to the invention is generally carried out at reduced pressure, below normal pressure, preferably at a pressure of 70 to 350 mbar, more preferably 80 to 200 mbar.

Step i) of the process according to the invention can in principle be carried out in any suitable reactor. Preference is given to using a tubular reactor, particularly preferably a multitubular reactor. Suitable tubular reactors are known to the person skilled in the art.

Usually, the gaseous formamide used in step i) is obtained by using liquid formamide in the heat exchanger, preferably a tube bundle heat exchanger, fine film evaporator or thin film evaporator under reduced pressure of generally 1 to 350 mbar, preferably 80 to 250 mbar and at temperatures of generally 100 is evaporated to 300 ° C, preferably 130 to 220 ° C. The formamide vapor is then brought to the reaction temperature in the actual dehydration reactor or in an additional upstream heat exchanger.

The average residence time on the catalyst in step i) of the process according to the invention is generally 0.01 to 0.5 s, preferably 0.05 to 0.2 s.

In step i), a product mixture containing hydrocyanic acid, water, unreacted formamide, ammonia, CO and high boilers is obtained. For the purposes of the present application, high boilers include in particular polymers such as polymeric HCN and diformamide and salts, e.g. Ammonium cyanide, to understand.

Suitable downstream heat exchangers are heat exchangers which are suitable for the condensation of a liquid containing formamide and water from the product mixture obtained in step i). Examples of suitable heat exchangers are tube bundle heat exchangers, plate heat exchangers, thermal sheets, etc.

Step ii)

Following the catalytic dehydration in step i), the condensation of a liquid containing formamide and water (condensate) takes place from the abovementioned product mixture. The condensate generally contains water, formamide, high boilers and possibly ammonia and hydrocyanic acid. Ammonia and hydrocyanic acid are generally present in the condensate only in small amounts of <0.5 wt .-%. Furthermore, in the condensation, a gaseous product stream is obtained, which generally contains hydrocyanic acid, water, ammonia and CO.

The amount of condensed liquid is in the partial condensation (based on the total mass flow of process gas) generally 1 to 20 wt.%, Preferably 3 to 15 wt .-% and particularly preferably 5 to 10 wt .-%.

The condensation in step ii) of the process according to the invention preferably takes place in the form that the product mixture formed in step i), in general has a temperature of 400 to 600 ° C, cooled to 10 to 150 ° C. This cooling is generally carried out in one or more heat exchangers or in a quench column.

The gaseous product stream containing generally hydrocyanic acid, water, ammonia and CO serves to obtain the desired hydrocyanic acid. In a preferred embodiment, ammonia is removed by extraction, for example by absorption in sulfuric acid, and hydrocyanic acid is subsequently obtained as condensate 2. The hydrocyanic acid can then be further purified, for example by fractional distillation.

Step iii)

In step iii), according to the invention, the continuous recycling of a portion of the condensed-out liquid and pumping back of this condensed liquid takes place on the condensation surfaces of the heat exchanger.

Generally, such a portion of the condensed-out liquid is recycled that the ratio of recycled condensate and raw gas (in parts by weight) is 1: 2 to 1:20, preferably 1: 3 to 1:15, and more preferably 1: 3 to 1:10 ,

With the method according to the invention long service lives of generally at least six months can be achieved. Thus, the inventive method is used to reduce the cleaning costs and shorten the downtime of a plant for the production of hydrogen cyanide. The following examples further illustrate the invention.

Examples Example 1 (according to the invention)

A 4.5 m reaction tube made of 1.4541 steel (V2A steel) with an inner diameter of 10 mm and an outer diameter of 12 mm is electrically brought to a constant outside temperature of 520 ° C. The reaction tube has a specific surface area of 400 m ² / m ³ . The internal pressure in the pipe is 150 mbar abs. and is generated by a vacuum pump.

In an upstream heat exchanger (evaporator), which is also under the reaction pressure, 1.3 kg / h of formamide are evaporated at 155 ° C and passed to the top of the reaction tube. In addition, at the connection between the heat exchanger and the reaction tube 13 NL air / h is fed.

At the end of the reaction tube is a shell-and-tube heat exchanger, which cools the process gas to 80 ° C in one stage. The cooling water temperature is 23 ° C.

In this procedure, 0.11 kg of a condensate 1 containing 67% by weight of formamide, 32% by weight of water, 0.1% by weight of NH ₃ and about 0.3% by weight of HCN are formed per hour. This condensate 1 is pumped with a circulation pump (0.5 kg / h) to the entry point of the heat exchanger. After a continuous period of 14 days, the apparatus is opened and no polymer coating can be found in the heat exchanger. All pipes are free.

Example 2 (comparative experiment)

A 4.5 m reaction tube made of 1.4541 steel (V2A steel) with an inner diameter of 10 mm and an outer diameter of 12 mm is electrically brought to a constant outside temperature of 520 ° C. The reaction tube has a specific surface area of 400 m ² / m ³ . The internal pressure in the pipe is 150 mbar abs..and is generated by a vacuum pump.

In an upstream heat exchanger, which is also under the reaction pressure, 1.3 kg / h of formamide are evaporated at 155 ° C and passed to the top of the reaction tube. In addition, 13 NL air / h is fed to the connection between the evaporator and the reaction tube.

At the end of the reaction tube is a shell-and-tube heat exchanger, which cools the process gas to 80 ° C in one stage. The cooling water temperature is 23 ° C.

A return of condensate as in the example of the invention does not take place. After a continuous run of 9 days, the apparatus had to be opened as a HCN polymer coating clogged the heat exchanger.

Claims (7)

1. A process for preparing hydrocyanic acid, which comprises

   i) catalytic dehydration of gaseous formamide in a reactor having a downstream heat exchanger having condensation surfaces to give a product mixture comprising hydrocyanic acid, water, formamide, ammonia, CO and high boilers;

   ii) partial condensation of a liquid phase comprising formamide and water and possibly ammonia and hydrocyanic acid in small amounts of < 0.5% by weight to give a gaseous product stream which comprises hydrocyanic acid, water, ammonia and CO, from the product mixture obtained;

   iii) continuous recirculation of part of the condensed-out liquid and pumping of this condensed-out liquid back onto the condensation surfaces of the heat exchanger, with the entire condensation surfaces of the downstream heat exchanger being continually wetted with liquid.
2. The process according to claim 1, wherein the condensed liquid is fed onto the condensation surfaces of the heat exchanger at one or more points.
3. The process according to claim 1 or 2, wherein highly sintered shaped bodies made up of from 50 to 100% by weight of aluminum oxide and from 0 to 50% by weight of silicon dioxide, shaped chromium-nickel stainless steel bodies, packings comprising steel and/or iron oxide on a support material are used as catalysts in step i) and/or the inside of the wall of the reactor is made of steel and serves as catalyst.
4. The process according to any of claims 1 to 3, wherein step i) is carried out in the presence of atmospheric oxygen.
5. The process according to any of claims 1 to 4, wherein step i) is carried out at a temperature of from 350 to 650°C.
6. The process according to any of claims 1 to 5, wherein step i) is carried out at a pressure of from 70 to 350 mbar.
7. The process according to any of claims 1 to 6, wherein the condensation in step ii) is effected by cooling the product mixture formed in step i) to from 10 to 150°C.